1) Machine Type Communication (MTC)
As the internet of things is emerging, a support of MTC in a Long Term Evolution (LTE) system has been increasingly recognized. An enhanced physical layer project for MTC has been set up in the 3GPP Release 13. An MTC device (MTC terminal) may have a part of various Machine to Machine (M2M) communication characteristics, e.g., low mobility, a small amount of data to be transmitted, insensitivity to a communication delay, extremely low power consumption as required, and other characteristics. In order to lower a cost of the MTC terminal, a type of terminal supporting only a 1.4 MHz radio frequency bandwidth in the uplink and the downlink will be newly defined.
In the existing networks, the operators have identified that for a terminal operating in some scenario, e.g., a terminal operating in a basement, in a shopping mall, or at a corner of a building, a radio signal is seriously shielded, and the signal is greatly attenuated, so the terminal may not communicate with the network, but if a coverage area of the network is extended in such a scenario, then a cost of deploying the network will be greatly increased. Some test has showed that the existing coverage area needs to be enhanced to some extent. A feasible practice to enhance the coverage area is to apply repeated transmission or other similar technologies to the existing channels, and theoretically tens or hundreds of repeated transmission may be performed over the existing physical channels for some coverage gain.
2) Random Access Mechanisms in the LTE System
There are generally the following several reasons for a random access in the LTE system.
An access from a radio resource control idle RRC_IDLE state (also referred to as an initial access); a radio link fails, and RRC connection reestablishment is initiated (also an initial access); a random access is required for a handover procedure; there are downlink data arriving at a UE in an RRC_CONNECTED state; and there are uplink data arriving at a UE in an RRC_CONNECTED state.
In the case that there are downlink data arriving, and in the case that there is a handover, if there is a dedicated preamble, then a contention-free random access may be performed, where a process of the contention-free random access is as illustrated in FIG. 1, and generally includes the following three operations.
In the operation 1, an eNB transmits a message Msg0 to a UE.
The base station allocates for the UE a dedicated Random Access Preamble Index (ra-PreambleIndex) for a contention-free random access, and a Physical Random Access Channel (PRACH) resource Index (ra-PRACH-MaskIndex), where the values of ra-PRACH-MaskIndex are as depicted in Table 1. For a contention-free random access due to downlink data arriving, such information is carried using PDCELLH, and for a contention-free random access due to a handover, such information is carried using a handover order; and the information is transmitted to the UE via the message Msg0.
In the operation 2, the UE transmits a message Msg1 to the eNB.
The UE transmits a specified dedicated preamble to the eNB over the specified PRACH resource according to the ra-PreambleIndex and the ra-PRACH-MaskIndex indicated in the Msg0. The eNB calculates an uplink Timing Advance (TA) according to the Msg1 upon reception of the Msg1.
In the operation 3, the eNB transmits a message Msg2 to the UE.
The eNB transmits a random access response to the UE via the Msg2, where the random access response includes information about the timing advance, and notifies the UE of the timing advance for subsequent uplink transmission.
A contention random access can be applicable to random accesses due to all other random access reasons, and a procedure of the contention random access is as depicted in FIG. 2, and generally includes the following four operations.
In the operation 1, the UE transmits a message Msg1 to the eNB.
The UE selects a preamble for a random access, and a PRACH resource, and transmits the selected preamble for a random access to the eNB via the Msg1 over the PRACH resource.
In the operation 2, the eNB transmits a message Msg2 to the UE.
The eNB receives the preamble, calculates an uplink Timing Advance (TA), and transmits a random access response to the UE via the Msg2, where the random access response at least includes information about the timing advance, and information about an uplink scheduling grant for a message Msg3.
In the operation 3, the UE transmits the message Msg3 to the eNB.
The UE transmits in the uplink via the Msg3 over the UL grant specified in the Msg2, where different information items are transmitted in the uplink via the Msg3 due to different random access reasons, for example, an RRC Connection Setup Request is transmitted via the Msg3 for an initial access.
In the operation 4, the eNB transmits a message Msg4 to the UE.
The Msg4 is a contention resolution message, and the UE may determine from the Msg4 whether the random access succeeds.
When there is a handover, or there are downlink data arriving, the network side may trigger the UE to initiate a contention-free random access procedure, when there are insufficient PRACH resources at the network side, the network side may alternatively trigger the UE via a message Msg0 to initiate a contention random access directly, where ra-PreambleIndex is particularly set to 000000.
TABLE 1Values of PRACH Mask IndexPRACHMask indexAllowed PRACH (FDD)Allowed PRACH (TDD)0AllAll1PRACH resource index 0PRACH resource index 02PRACH resource index 1PRACH resource index 13PRACH resource index 2PRACH resource index 24PRACH resource index 3PRACH resource index 35PRACH resource index 4PRACH resource index 46PRACH resource index 5PRACH resource index 57PRACH resource index 6Reserved8PRACH resource index 7Reserved9PRACH resource index 8Reserved10PRACH resource index 9Reserved11Every, in the time domain, even PRACHEvery, in the time domain, even PRACHopportunityopportunity1st PRACH Resource Index in sub-frame1st PRACH Resource Index in sub-frame12Every, in the time domain, odd PRACHEvery, in the time domain, odd PRACHopportunityopportunity1st PRACH Resource Index in sub-frame1st PRACH Resource Index in sub-frame13Reserved1st PRACH resource index in sub-frame14Reserved2nd PRACH resource index in sub-frame15Reserved3rd PRACH resource index in sub-frame
3) A Random Access Channel (RACH) Transmission Mechanism in Low-Complexity MTC (LC MTC)
In order to improve the performance of transmission in an enhanced coverage area, repeated transmission needs to be performed for a number of times. For the random access procedure, the scheduling signaling of the messages in the respective operations, the messages per se, and their corresponding feedback information need to be transmitted repeatedly. The same category of message is required to be transmitted repeatedly for different numbers of time at different Coverage Enhancement (CE) Levels, so it has been ascertained at the RAN2#90 session that there are different transmission resources (PRACH resources) for transmitting the Msg1 at the different CE levels, and the UE may determine the current CE LEVEL of the UE according to its current circumstance (e.g., Radio Resource Management (RRM) measurement, etc.), and then select a PRACH resource at the CE level to transmit the Msg1.
For a random access procedure initiated by the UE on its own initiative, when a success ratio of transmitting a preamble at the same CE level reaches a pre-configured number of transmissions, but the preamble still fails to be transmitted, the UE need to raise the CE level by one level, and select a PRACH resource at the one-level higher CE level to transmit the preamble.
Apparently in the existing transmission process during a random access in an enhanced coverage area, the PRACH resource for a random access initiated by the UE on its own initiative is dependent upon the current CE level of the UE; and in a random access procedure initiated by the UE as a result of being triggered by the network side, the PRACH resource is determined by the network side. Since the network side triggers the UE to initiate a random access procedure, primarily for downlink data arriving or a handover, these two procedures are common in that no data have been transmitted for a long period of time, or no data have ever been transmitted, between the UE and the target eNB. In the case that no data have been transmitted for a long period of time, it is difficult for the eNB at the network side to get timely and accurate knowledge of the current CE level of the UE. There has been absent so far a specific solution to selecting a PRACH resource by the UE in this case.